1. Field of the Invention
The present invention relates generally to circuits, and more specifically to power sequencing in multiple power domain circuits.
2. Background Art
Analog circuit applications that require high speed and/or low distortion often necessitate a higher supply voltage as compared to slower or noisier applications. The higher supply voltage can give devices of an application larger head room and lower junction capacitance, resulting in better linearity and faster speed. However, the supply voltage is usually limited so as to not exceed the breakdown voltage of semiconductor components of the analog circuit. With cascaded devices, though, the supply voltage is not necessarily limited to a value below the breakdown voltage. As long as the voltage across each cascaded device does not exceed the breakdown voltage of the respective device, the supply voltage can be increased.
Analog circuits usually have more than one cascade of devices from supply to ground. Thus, the supply voltage can be larger than the breakdown voltage of a single device. Although high speed or high performance circuits may require a high voltage supply, slower portions of the circuit, such as bandgap or bias blocks, can be operated using lower supply voltages. To save power and area, two or more supply voltages may be used in a single chip.
However, circuits with multiple supply voltages generally have problems when power is switched on. If a lower voltage supply is turned on before a higher voltage supply, devices may burn out due to current overflow. If a higher voltage supply is turned on before a lower voltage supply, devices may exceed breakdown voltage because no reference is available upon which to base biasing of the devices.
What is needed, then, is a system and method to enable start-up of circuits having multiple power supplies without burning out or exceeding a breakdown voltage.